Public health threats of diminished treatment of onsite sewage
Authors: Mary G Lusk
Journal: The Lancet Planetary Health (2022)

The sufficient treatment of human waste is essential for public health. The UN estimates that 24% of global domestic wastewater (human excreta and household wastewater) is generated by homes with onsite sewage treatment and disposal systems (OSTDS), which are commonly referred to as septic systems.1UN-Habitat and WHOProgress on wastewater treatment—global status and acceleration needs for SDG indicator 6.3.1. United Nations Human Settlements Programme and World Health Organization, Geneva2021Google Scholar An OSTDS is a self-contained underground sewage treatment system that is normally located at the same place the sewage is produced, such as an individual residence. OSTDS have two main components: an underground tank (the septic tank) that collects wastewater and a soil treatment area (often called the drainfield or leachfield) in which effluent from the tank is dispersed through the underlying soil. Underground, out of sight, and often out of mind, OSTDS rely on various physical and biogeochemical processes in the underlying soil to reduce, transform, absorb, or remove contaminants from domestic wastewater.The unseen work of the OSTDS soil is a crucial protector of the groundwater that lies beneath it. This is often the same groundwater that is used for drinking water wells on the same or adjacent property. Thus, the proper functioning of OSTDS helps prevent wastewater-borne illnesses and ensures safe and adequate sanitation for billions of people worldwide. However, a recent compilation of data by the UN estimates that only 48% of global OSTDS are safely treating the wastewater flows they collect.1UN-Habitat and WHOProgress on wastewater treatment—global status and acceleration needs for SDG indicator 6.3.1. United Nations Human Settlements Programme and World Health Organization, Geneva2021Google Scholar This finding is alarming for sanitation and planetary health. Adding to this concern is that much of what makes an OSTDS treat human waste safely is the sufficient depth of soil above the local water table (usually at least 60 cm) and that climate change-induced rising groundwater levels in coastal regions could further diminish the proportion of human waste safely treated by OSTDS throughout the world.2Cox AH Dowling MJ Loomis GW Engelhart SE Amador JA Geospatial modeling suggests threats from stormy seas to Rhode Island's coastal septic systems.J Sustain Water Built Environ. 2020; 6: 04020012Crossref Scopus (5) Google Scholar, 3Cox AH Loomis GW Amador JA Preliminary evidence that rising groundwater tables threaten coastal septic systems.J Sustain Water Built Environ. 2019; 5: 04019007Crossref Scopus (11) Google Scholar Septic tank effluent contains numerous contaminants originating from faecal matter, urine, and household wastewater (figure). Most contaminant treatment and removal in an OSTDS occurs as tank effluent percolates through the soil, and it is especially important for that soil to be unsaturated (ie, have some pore spaces between soil grains filled with air instead of water). Air-filled soil pores slow effluent transport to groundwater, enable effluent interaction with soil, and allow for processes such as pathogen absorption to soil surfaces and subsequent pathogen die-off and degradation of harmful organic compounds by aerobic soil bacteria. Simply put, sea level rise also causes a rise in groundwater levels for adjacent coastal areas, and a rising water table leads to diminished OSTDS effluent contact and treatment with unsaturated soil. In a coastal area near Miami, FL, groundwater levels have increased approximately 2·8 mm/year since at least 1979, which is consistent with local sea level rise of 2·4 mm/year during the same time period.4Sukop MC Rogers M Guannel G Infanti JM Hagemann K High temporal resolution modeling of the impact of rain, tides, and sea level rise on water table flooding in the Arch Creek basin, Miami-Dade County Florida USA.Sci Total Environ. 2018; 616–617: 1668-1688Crossref PubMed Scopus (21) Google Scholar An OSTDS drainfield installed in 1979 under this scenario will have lost 12 cm of crucial unsaturated soil (20% of the 60 cm recommended for optimal waste treatment).A full discussion of the mechanisms by which wastewater contaminants can be removed by OSTDS soils is provided by Lusk and colleagues, 2017.5Lusk M Toor G Yang Y Mechtensimer S De M Obreza T A review of the fate and transport of nitrogen, phosphorus, pathogens, and trace organic chemicals in septic systems.Crit Rev Environ Sci Technol. 2017; 47: 455-541Crossref Scopus (59) Google Scholar In one example of these mechanisms, viruses, which are too small to be filtered out by physical straining, are often removed from OSTDS effluent when the viral outer protein covering is adsorbed to the surfaces of soil grains, after which the virus is inactivated or degraded over time. Virus adsorption to soil surfaces is possible because the protein coverings have an electrical charge that allows them to bind to surfaces outside of their host cells, but this process is reversed when OSTDS soils are saturated by rising groundwater or heavy rains, so that viruses instead desorb from soils while still infective.Considering the risks associated with poorly treated OSTDS waste (figure), the planetary health co-benefits of preventing or mitigating rising groundwater under OSTDS soils are clear. Although a global accounting of OSTDS strictly in coastal areas prone to rising sea levels is absent, we know that, in Asia, 21–41% of household wastewater is generated by residences with OSTDS,1UN-Habitat and WHOProgress on wastewater treatment—global status and acceleration needs for SDG indicator 6.3.1. United Nations Human Settlements Programme and World Health Organization, Geneva2021Google Scholar and that people most likely to be affected by sea level rise are in low-income and middle-income countries,6Kulp SA Strauss BH New elevation data triple estimates of global vulnerability to sea-level rise and coastal flooding.Nat Commun. 2019; 10: 4844Crossref PubMed Scopus (421) Google Scholar supporting the interconnections between climate change, sanitation, and planetary health. Densely populated and urban parts of the eastern USA have already reported loss of OSTDS functionality because of rising groundwater (eg, Boston, New York City, and Miami), and it is likely that this is also occurring in other coastal regions, including those where socioeconomic disparities exacerbate vulnerability, such as the Small Island Developing States.3Cox AH Loomis GW Amador JA Preliminary evidence that rising groundwater tables threaten coastal septic systems.J Sustain Water Built Environ. 2019; 5: 04019007Crossref Scopus (11) Google Scholar, 7Scandurra G Romano A Ronghi M Carfora A On the vulnerability of Small Island developing states: a dynamic analysis.Ecol Indic. 2018; 84: 382-392Crossref Scopus (38) Google Scholar As we build climate change resilience in global communities, there is a need to remember health risks associated with diminished treatment of OSTDS waste. Actions could include identifying and mapping OSTDS hotspots that correspond with regions of rising groundwater for prioritising action; local policies that require regular monitoring of OSTDS functionality so loss of unsaturated soil can be detected early; funding and regulatory mechanisms to upgrade or remove failing OSTDS or to replace them with central sewer services, and financial assistance to low-income countries to achieve this; engineering research that will lead to new and economically viable OSTDS technology with enhanced contaminant removal capabilities; and guidance to communities about how to identify OSTDS vulnerable to climate change and how to share that knowledge in a way that translates into appropriate action. The sufficient treatment of human waste is essential for public health. The UN estimates that 24% of global domestic wastewater (human excreta and household wastewater) is generated by homes with onsite sewage treatment and disposal systems (OSTDS), which are commonly referred to as septic systems.1UN-Habitat and WHOProgress on wastewater treatment—global status and acceleration needs for SDG indicator 6.3.1. United Nations Human Settlements Programme and World Health Organization, Geneva2021Google Scholar An OSTDS is a self-contained underground sewage treatment system that is normally located at the same place the sewage is produced, such as an individual residence. OSTDS have two main components: an underground tank (the septic tank) that collects wastewater and a soil treatment area (often called the drainfield or leachfield) in which effluent from the tank is dispersed through the underlying soil. Underground, out of sight, and often out of mind, OSTDS rely on various physical and biogeochemical processes in the underlying soil to reduce, transform, absorb, or remove contaminants from domestic wastewater. The unseen work of the OSTDS soil is a crucial protector of the groundwater that lies beneath it. This is often the same groundwater that is used for drinking water wells on the same or adjacent property. Thus, the proper functioning of OSTDS helps prevent wastewater-borne illnesses and ensures safe and adequate sanitation for billions of people worldwide. However, a recent compilation of data by the UN estimates that only 48% of global OSTDS are safely treating the wastewater flows they collect.1UN-Habitat and WHOProgress on wastewater treatment—global status and acceleration needs for SDG indicator 6.3.1. United Nations Human Settlements Programme and World Health Organization, Geneva2021Google Scholar This finding is alarming for sanitation and planetary health. Adding to this concern is that much of what makes an OSTDS treat human waste safely is the sufficient depth of soil above the local water table (usually at least 60 cm) and that climate change-induced rising groundwater levels in coastal regions could further diminish the proportion of human waste safely treated by OSTDS throughout the world.2Cox AH Dowling MJ Loomis GW Engelhart SE Amador JA Geospatial modeling suggests threats from stormy seas to Rhode Island's coastal septic systems.J Sustain Water Built Environ. 2020; 6: 04020012Crossref Scopus (5) Google Scholar, 3Cox AH Loomis GW Amador JA Preliminary evidence that rising groundwater tables threaten coastal septic systems.J Sustain Water Built Environ. 2019; 5: 04019007Crossref Scopus (11) Google Scholar Septic tank effluent contains numerous contaminants originating from faecal matter, urine, and household wastewater (figure). Most contaminant treatment and removal in an OSTDS occurs as tank effluent percolates through the soil, and it is especially important for that soil to be unsaturated (ie, have some pore spaces between soil grains filled with air instead of water). Air-filled soil pores slow effluent transport to groundwater, enable effluent interaction with soil, and allow for processes such as pathogen absorption to soil surfaces and subsequent pathogen die-off and degradation of harmful organic compounds by aerobic soil bacteria. Simply put, sea level rise also causes a rise in groundwater levels for adjacent coastal areas, and a rising water table leads to diminished OSTDS effluent contact and treatment with unsaturated soil. In a coastal area near Miami, FL, groundwater levels have increased approximately 2·8 mm/year since at least 1979, which is consistent with local sea level rise of 2·4 mm/year during the same time period.4Sukop MC Rogers M Guannel G Infanti JM Hagemann K High temporal resolution modeling of the impact of rain, tides, and sea level rise on water table flooding in the Arch Creek basin, Miami-Dade County Florida USA.Sci Total Environ. 2018; 616–617: 1668-1688Crossref PubMed Scopus (21) Google Scholar An OSTDS drainfield installed in 1979 under this scenario will have lost 12 cm of crucial unsaturated soil (20% of the 60 cm recommended for optimal waste treatment). A full discussion of the mechanisms by which wastewater contaminants can be removed by OSTDS soils is provided by Lusk and colleagues, 2017.5Lusk M Toor G Yang Y Mechtensimer S De M Obreza T A review of the fate and transport of nitrogen, phosphorus, pathogens, and trace organic chemicals in septic systems.Crit Rev Environ Sci Technol. 2017; 47: 455-541Crossref Scopus (59) Google Scholar In one example of these mechanisms, viruses, which are too small to be filtered out by physical straining, are often removed from OSTDS effluent when the viral outer protein covering is adsorbed to the surfaces of soil grains, after which the virus is inactivated or degraded over time. Virus adsorption to soil surfaces is possible because the protein coverings have an electrical charge that allows them to bind to surfaces outside of their host cells, but this process is reversed when OSTDS soils are saturated by rising groundwater or heavy rains, so that viruses instead desorb from soils while still infective. Considering the risks associated with poorly treated OSTDS waste (figure), the planetary health co-benefits of preventing or mitigating rising groundwater under OSTDS soils are clear. Although a global accounting of OSTDS strictly in coastal areas prone to rising sea levels is absent, we know that, in Asia, 21–41% of household wastewater is generated by residences with OSTDS,1UN-Habitat and WHOProgress on wastewater treatment—global status and acceleration needs for SDG indicator 6.3.1. United Nations Human Settlements Programme and World Health Organization, Geneva2021Google Scholar and that people most likely to be affected by sea level rise are in low-income and middle-income countries,6Kulp SA Strauss BH New elevation data triple estimates of global vulnerability to sea-level rise and coastal flooding.Nat Commun. 2019; 10: 4844Crossref PubMed Scopus (421) Google Scholar supporting the interconnections between climate change, sanitation, and planetary health. Densely populated and urban parts of the eastern USA have already reported loss of OSTDS functionality because of rising groundwater (eg, Boston, New York City, and Miami), and it is likely that this is also occurring in other coastal regions, including those where socioeconomic disparities exacerbate vulnerability, such as the Small Island Developing States.3Cox AH Loomis GW Amador JA Preliminary evidence that rising groundwater tables threaten coastal septic systems.J Sustain Water Built Environ. 2019; 5: 04019007Crossref Scopus (11) Google Scholar, 7Scandurra G Romano A Ronghi M Carfora A On the vulnerability of Small Island developing states: a dynamic analysis.Ecol Indic. 2018; 84: 382-392Crossref Scopus (38) Google Scholar As we build climate change resilience in global communities, there is a need to remember health risks associated with diminished treatment of OSTDS waste. Actions could include identifying and mapping OSTDS hotspots that correspond with regions of rising groundwater for prioritising action; local policies that require regular monitoring of OSTDS functionality so loss of unsaturated soil can be detected early; funding and regulatory mechanisms to upgrade or remove failing OSTDS or to replace them with central sewer services, and financial assistance to low-income countries to achieve this; engineering research that will lead to new and economically viable OSTDS technology with enhanced contaminant removal capabilities; and guidance to communities about how to identify OSTDS vulnerable to climate change and how to share that knowledge in a way that translates into appropriate action. I declare no competing interests.